This is a fundamental law of economics. At the micro level, economics looks at each discrete decision you make, and decisions always involve trade-offs. You want to buy ice cream. You decide the ice cream is worth more than your money, and the ice cream is worth more than something else you could do with your money, such as buying some chocolate. If you think for awhile, you'll realize everything is a trade-off. Resources, such as money and time, are finite.
That answers half your question, and the other half is a bit trickier to explain. Economists separate statements into normative and positive ones. A positive statement just describes what is. A normative one says what should be. Using ice cream as an example, a positive statement is "I bought ice cream." A normative statement is "Buying ice cream is good." With that in mind, an economic solution is necessarily normative because it says "This choice is the solution." Good economists steer clear of normative statements and prefer to say "These are your choices, and these are the outcomes." Given that positive view, you can see that you have a set of trade-offs with each choice and not a conclusive solution.
Graduate economics courses